1. Field of the Invention
The present invention generally relates to the field of photovoltaic devices. Specifically, the present invention relates to the areas of dye sensitized solar cells (DSSCs).
2. Description of Related Art
As global climate change becomes an issue of increasing concern, cutting greenhouse gas emissions from energy production has become top priority. Solar energy is currently thought to hold immense potential as a reliable clean energy source. Effective utilization of solar energy relies on the development of efficient photovoltaic devices including dye sensitized solar cells (DSSCs).
Nanocrystalline dye-sensitized solar cells (DSSC) are appealing devices for solar-electric energy conversion because of their potentially low cost, and simple process in comparison to silicon-based photovoltaics. In a classic DSSC device configuration, the incident solar light first penetrates the device through a piece of transparent conducting oxide (TCO) glass, on which a thick layer of interconnected semiconducting nanoparticles (NP) such as TiO2 are coated to provide a large internal surface area for anchoring the light-harvesting dye molecules, whose lowest unoccupied molecular orbital matches the conduction band edge of the nanoparticles. The dye molecules are excited by the incident photons, leading to electron-hole pairs (excitons), of which the electrons are quickly injected into the conduction band of the semiconducting nanoparticles and subsequently collected by the TCO anode, while the holes in the highest unoccupied molecular orbital are refilled by electrons from the cathode via redox species, e.g. an I−/I3− couple in an electrolyte that interpenetrates the nanoparticle network.
The trap-limited electron transport in the nanoparticle network often leads to low electron-collection efficiency, thereby leading to low energy conversion efficiency, inherently due to the defect states at the particle-particle interface or in the bulk of the nanoparticles, where the congested electrons are recombined with the oxidized dye molecules or redox species such as I3−. On the other hand, the photocurrent is also influenced by the diffusion of the I−/I3− couple in electrolyte.
The overall efficiency (η) of a solar cell is calculated from η=(FF×|Jsc|×Voc)/I, where Jsc is the short circuit photocurrent density, Voc is the open-circuit photovoltage, FF is the fill factor of the cell and I is the light intensity (I=0.1 W/cm2 for one sun at AM 1.5 G). In addition to maximizing the overlap of the absorption window of dyes and the solar spectrum through the discovery of new dyes, Jsc can also be improved by thickening the TiO2 nanoparticle film for increased optical density. However, a dilemma arises in that the electron diffusion length, typically 10 μm in TiO2 nanoparticle networks, limits the useful TiO2 nanoparticle film thickness.
Many efforts are focused on improving the electron diffusion length by using nanostructured anodes with a higher degree of order than the random fractal-like assembly of nanoparticles. Those semiconducting nanostructures include nonporous channels, nanotubes, or even single-crystalline nanowires that are all aligned in parallel to each other and vertically with respect to the TCO glass.
For example, enhanced electron transfers have been reported in various n-type semiconducting layers consisting of arrays of one-dimensional (1-D) nanostructures including ZnO nanowires and nanotubes, TiO2 nanotubes, etc. These highly ordered 1-D semiconducting nanostructures provide an ordered pathway for electrons percolating to the collecting anode, in contrast to the highly disordered electron pathway found in a nanoparticulate layer that can lead to significant scattering of free electrons at the particle-particle interfaces. In addition, if the radius of the 1-D n-type semiconductors is large enough, an upward band bending at the semiconductor surface can form, which suppresses the adverse back electron transfers from the semiconductor to the electrolyte or to the oxidized dyes. This is because the Fermi level of an n-type semiconductor is typically higher than the redox potential of the electrolyte. To equilibrate the two electron levels, electrons flow from the semiconductor into the electrolyte. As a result, there is a built-in circular electric field from the surface of the semiconductor nanowires towards their centers. This internal electric field pulls the injected electrons towards the center of the wire and reduces the interception of the electrons by the electrolyte around the surface of the wire. The suppression of back electron transfer improves the current density of the cells.
However, so far, none of these ordered 1-D semiconductor-based DSSCs have achieved an efficiency exceeding that of conventional TiO2 nanoparticle-based DSSCs. This is because many other device parameters are often interlinked, which can offset or reduce the improvements available through the new features. One particular problem is the diametric opposing effect resulting from increasing the length of the ZnO nanowires. On one hand, longer wires exhibit higher short circuit current (Jsc) due to the increased surface area and thus higher dye loading. On the other hand, longer wires lead to higher series resistance, thus lowering the fill factor (FF). Narrower, thus denser nanowires appear as a potential approach to overcome this problem. However, if the Debye-Hückel screening length exceeds the wire radius, reducing the diameter of the wires can eliminate the upward band bending at the wire's surface, an advantage of 1-D semiconductor elucidated above. Typically, depending on the carrier density and the electrolyte, the width of the depletion layer can extend to tens of nanometers into the ZnO wires.
Another fundamental bottleneck that substantially impedes the advantages brought about by 1-D semiconductor photoanode is the slow hole transport that is carried by redox species through mass transport in electrolyte. In all DSSCs that are based on the I−/I3− as redox shuttle, the cathode is essentially a planar platinized TCO that is separated from the semiconductor layer by the electrolyte. Pt is an indispensable catalyst for efficient reduction of I3− to I−. The Pt cathode is typically 20˜40 μm apart from the top of the semiconductor layer, defined by a polymer spacer as sealer. The diffusion coefficient of I3− in the electrolyte is less than 10-4 cm2/s at room temperature, which is 2˜3 orders of magnitude slower than the electron diffusion coefficient in 1-D semiconductor nanowires (>10-2 cm2/s for ZnO nanowires, for example). Thus, the synchronism of charge carriers (both electrons and holes) transport can no longer be established. As a result, many adverse back electron transfers will take place including the recombination of the electron the semiconductors with dye+ and I3− as well as the formation of dye+−I3− complex.
These dilemmas make it especially necessary to explore further innovations to drastically improve this fascinating photovoltaic device. The aim is (1) to increase the effective semiconductor thickness in the conventional nanoparticle-based DSSC without exceeding the electron diffusion length, (2) to increase the surface roughness of 1-D nanostructured semiconductor without significantly increasing the length, and thus the series resistance; (3) to alter the route of I−/I3− diffusion pathways to catch up with the fast electron transport in the 1-D ordered semiconductor-based photoanode so that the electron/hole transport can be synchronized to exert the advantage of fast electron transport in 1-D photoanode; and (4) to enhance the interfacial rectifying effect to suppress the back electron transfer.